Endothelial cell loss after penetrating keratoplasty leads to graft failure. The critical limit of the endothelial cell count for corneal decompensation is thought to be 700 cells/mm2. Clinical specular microscopy indicates that inspite of low endothelial cell density, the corneal graft can be maintained in a relatively dehydrated state. This study reports 4 cases of clear corneal graft having low endothelial cell count. The low endothelial cell count was associated with polymegathism and pleomorphism; inspite of this, the clarity of the graft was maintained.

The primary goal after corneal transplantation is preservation of a clear corneal graft. Endothelial cell loss after penetrating keratoplasty (PK) is an ongoing process. The prognosis of corneal grafts after PK has improved due to enhanced treatment and corneoscleral storage techniques. Studies indicate that endothelial cell loss is higher early and decreases 3–5 years after surgery. Despite a small degree of continuous cell loss, corneal grafts have a favorable prognosis for long-term clinical stability. The endothelial cells are able to sustain their function and maintain corneal deturgescence and clarity even if their number is decreased by 80%.[1] However, in the long run, this continuous decrease in endothelial cell count and their concomitant increase in size and shape are accompanied by a decreased ability to maintain and restore pump and barrier function. Ultimately, a point is reached when endothelial function fails and the cornea swells and becomes hazy leading to graft failure. Low endothelial cell count associated with clear corneal graft has been reported earlier.[2],[3],[4],[5],[6] This study reports 4 cases of clear graft with low endothelial cell count after PK.

Case Report

Four patients followed up for 1 year and presenting with clear corneal are included in this case series. Preoperative endothelial cell count of donor cornea was done by Eyebank specular microscope (Konan Keratoanalyzer EKA-98), and the endothelial cell count of the recipient was done with noncontact specular microscope (SP-2000P [Topcon]).

The recipient's age, gender, preoperative diagnosis, pre- and post-operative best-corrected visual acuity (BCVA), preoperative endothelial cell count, endothelial cell count after 1 year, and percentage of endothelial cell loss are shown in [Table 1]. The age of these 4 donors was between 37 and 55 years (mean: 49 ± 8.16 years). The age of these 4 recipients was between 35 and 52 years (mean: 43 ± 7.04 years). The BCVA on last visit was 6/18 in 1 case while other 3 had BCVA between 6/60 and 6/24. The endothelial cell count of these grafts ranged from 613 to 738 cells/mm 2 (mean ± SD: 671.5 ± 56.97). The anterior segment and specular microscope photographs of one case are shown in [Figure 1] and [Figure 2].

To obtain a long-term functional transparent graft, adequate amounts of corneal endothelial cell density (ECD) is required. During the early postoperative period after PK, there is significant decrease in hexagonal cell percentage which is accompanied by increase in the coefficient of variation of cell area and the mean cell area. Reports on specular microscopic study after PK have shown that at times, low ECD can maintain the graft in a relatively dehydrated state. Therefore, there is an enormous reserve of extra endothelial cells to withstand intraocular surgery, trauma, and disease.

Mishima [7] first described the substantial physiological reserve of the human corneal endothelium. Chronic endothelial decompensation has been reported when the central ECD declines to 400–700 cells/mm 2[7],[8] and below 1000 cells/mm 2.[9]

Pesko andVölker-Dieben [5] have reported two cases where the ECD was 500 cells/mm 2. Rao et al.[6] have reported one case of clear corneal graft with a cell count of 445 cells/mm 2. In the present study, endothelial cell count as low as 614 cells/mm 2 was recorded in case of acute infective keratitis, and the graft survived on follow-up of 1 year with adequate clarity in spite of 76.4% loss in endothelial cell count. In the other three patients, low counts of 637, 698, and 738 cells/mm 2 were found.

Various studies have reported low endothelial count and clarity of graft after different duration of study. Lass et al.[10] noted graft survival in many eyes with ECD below 500 cells/mm 2. At 5 years, there were 40 such grafts, with ECDs as low as 178 cells/mm 2. Abbott et al.[4] reported a mean ECD of 684 cells/mm 2 among grafts that were followed up for a mean of 17.4 years after PK, with ECDs as low as 320 cells/mm 2.

Graft survival is linked to the functionality of the endothelial cells. As endothelial cell loss occurs, neighboring cells enlarge resulting in polymegathism and pleomorphism. Our findings suggest that corneas with low ECD can remain clear. Various hypotheses explain the graft clarity. When endothelial cells enlarges, the area of intercellular spaces available for pump sites decreases, while the barrier to flow into the cornea increases.[8],[11] The balance of leakage of fluid into the corneal stroma and the endothelial pump does not depend on the ECD alone, thus making the minimum ECD for maintaining the corneal thickness and graft clarity variable.

Kus et al.[3] in their study had an endothelial count <700 cells/mm 2. He postulated that with such reduced ECD, the normal endothelial cells in a corneal graft are able to maintain corneal clarity in the thin graft. The absence of guttae in graft endothelium could indicate that the endothelial cells in corneal grafts, though reduced in number, are essentially healthy.[6] Despite the reduction in cell density, the dehydration function of the endothelium is sufficient to maintain corneal clarity which may be explained by the reduced pump-leak hypothesis.[2]

Bourne [11] reported significantly decreased ECD, deswelling rate, and endothelial permeability to fluorescein, in 12 eyes with corneal grafts, compared to normal controls. He opined that the decreased cross-sectional area of the intercellular space facing the anterior chamber was due to the decreased number of cells in the endothelial cell layer.

From our case report, it may be concluded that in spite of low endothelial cell count, corneal grafts have a favorable prognosis for clinical stability. This also implies that there is a substantial reserve of extra endothelial cells that help the graft to survive and maintain clarity. The low endothelial cell count was associated with polymegathism and pleomorphism; inspite of this, the clarity of the graft was maintained.